Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024	Dec. 31, 2022	Nov. 16, 2023
Ceded Credit Risk [Line Items]
Reinsurance recoverables, allowance for doubtful accounts	$ 0.0	$ 0.0	$ 4.7	$ 5.8	$ 4.7	$ 0.0
Amount of collateral under reinsurance agreement	1,212.2	1,212.2		1,157.9
Reinsurance recoverables	2,959.3	2,959.3		3,053.2
Paid loss recoverables in insurance recoverables	161.9	161.9		258.5
Losses and loss adjustment expenses	94.2	94.2	1,043.2	1,018.3	1,166.9
Loss & LAE		94.2	1,043.2	1,018.3	1,166.9
Net of amounts ceded to reinsurers		79.3	$ 607.3	825.3	$ 958.8
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables	945.0	945.0		836.1
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments | Ceded Credit Risk, Unsecured | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables	$ 465.1	$ 465.1		$ 367.3